1. Field of Invention
The present invention relates to a metal oxide semiconductor (MOS) device, and more generally to a MOS device for electrostatic discharge (ESD) protection.
2. Description of Related Art
ESD is a phenomenon in which charges are accumulated on a non-conductor or a non-grounded conductor and then suddenly transfer and discharge through a discharge path. The circuits in ICs can be damaged by ESD. For example, a charge carrying object, such as a human body, a machine for packaging ICs or an apparatus for testing ICs, can discharge to an IC chip when the charge carrying object is in contact with the IC chip. Thus, the IC chip is damaged or broken by the transient power of ESD.
The conventional ESD protection device can be constructed with an ESD clamp circuit. Alternatively, the ESD protection device can be constructed with a silicon controlled rectifier (SCR) without using an ESD clamp circuit. However, it is more difficult to trigger a SCR. That is, the SCR requires a higher trigger voltage. Therefore, it is observed frequently that the device in the circuit is damaged before the SCR is activated. Accordingly, the ESD protection function can not be effectively achieved.